The invention relates to electronic ballasts for operating discharge lamps such as fluorescent lamps, and in particular to such ballasts having a minimum number of active components.
Most magnetically coupled self-oscillating inverters are manufactured in large quantities for sale in a highly competitive market. Half-bridge inverters are widely used because they have a relatively low parts count. Such inverters may be classified into two groups: those using a current transformer having a saturable core, generally together with power BJT's (bipolar junction transistors); and those using a current transformer having a linear core, generally together with MOSFETs (metal oxide semiconductor field effect transistors). As those of ordinary skill will recognize, in this context a linear core is one in which operation is over a region having a curved B-H characteristic, rather than a sharp B-H characteristic; that is, at all times the flux level is such that a significant increase in magnetizing current will be accompanied by a significant increase in flux level.
U.S. Pat. No. 5,608,295 discloses such a ballast circuit having a linear core resonance inductor in a series connected load circuit between the inverter output and signal ground. The inductor has secondary windings connected to the control electrodes of the switching transistors. The load circuit includes a tuning capacitor C8 and the primary winding of a matching transformer. Fluorescent lamps are connected to a secondary winding or windings of the matching transformer, and one or two additional tuning capacitors are connected across the lamps. A feedback conductor is connected between a tap on the primary winding of the matching transformer and a node between two 47 nf feedback capacitors which are connected in series across the AC input to a full wave bridge rectifier circuit. This patent teaches that the low frequency (power line) input current modulates in amplitude the high frequency feedback current which works as a carrier in order to transfer the low frequency input current through the bridge rectifier over most of the low frequency cycle. Accordingly it is clear that the line current is still discontinuous.